I heard this some time back from journalist photographers soon after I returned to India and pondered taking photo-journalism as a career. When I thought over the science behind it, it made sense.
If you are going to go out for a long time, eat peanut jaggery cakes (chikki). The jaggery converts into blood glucose pretty quick (not instantaneous such as honey though), so will provide energy for immediate need. The peanut provides fat that converts to blood glucose taking significantly longer time (for digestion) and offers significantly high quantity of calories. So this provides a sustained energy (though will not start immediately). Makes sense to me.
Having said that:
1. Fruits take about 1to 1.5 hours for digestion (and thus conversion to blood glucose).
2. Processed foods take anything from 4 hours to 6 hours to process
3. Digestive system needs greater amounts of oxygen for digestion. If one notices, breathing becomes faster soon after a meal and is least when on empty stomach such as in early morning. That is why soon after a heavy lunch, the oxygen supplied to brain is less and we feel sleepy. If this higher quantity of oxygen is denied to digestive system, it can lead to (a) longer period for digestion, (b) indigestion, and (c) acidity if continued for prolonged periods.
4. When we are exercising, muscles need more energy to perform. This is provided by mitocondria in cells by burning blood glucose and converting to energy. To enable burning, mitocondria needs oxygen. Thus when we exercise, we typically tend to breathe more heavily so there is more oxygen intake.
5. So It is imperative that body is provided some period of rest soon after a meal to allow for more oxygen intake. For if you are immediately on exercise after food, you are not providing enough oxygen for digestive system (remember digestive system drives everything in the body in long run).
Energy bars are easily converted to blood glucose due to high amount of polished and easily digestible food, lacking fiber and other natural nutrients. So these provide energy in shorter durations. However since these do not provide nutrients in a well balanced natural form, these are not recommended for prolonged use; can lead to diabetes during forties or sooner. There is enough evidenced research all over the world asking people to minimize/avoid processed and junk food. PLEASE don’t do this regularly.
To conclude: like Rajesh recommends, have good food during previous day and well balanced breakfast consisting all of carbohydrates, proteins, fats (in that order of proportion) and vitamins…..WELL BEFORE you go on a ride. The food should have been nearly digested before you go out. If you do not have so much time, have some quick digesting energy providers, namely fruits. You may carry and have chikki on and off during rides.
Hydration: yes, if you are thirsty, you are already late, damaged enough. Need to replenish with simple water just too often (amount varies based on perspiration nature, climate and such).